Mutations of various types are rare events that occur in all proliferating cell populations. Those affecting the germ line may be expressed by progenyY; those affecting somatic cells are more occult and therefore have received relatively little attention. Some of the first evidence that mutation occurs in mammalian somatic cells was cytogenetic: occasional cells dividing in vitro exhibits chromosome aberrations, some of which are heritable. Certain rare cancer-predisposing human genetic disorders have been discovered in which microscopically detectable aberrations are increased, Bloom's syndrome (BS) being the prototype. The types of aberrations in BS resemble those in normal cells, except for their increased frequency Cytogenetics has provided only the broadest suggestion of the nature of the mutations that occur spontaneously in proliferating somatic cells. The work proposed applies recombinant DNA technology to analyze these mutations, including those resulting from recombinational events, and to a certain extent to define them. BS cells will be used as the experimental model because of the increased frequency of mutations, as shown cytogenetically. DNA of single cells (i.e., of clones and subclones) of lymphoblastoid cells will be analyzed using a panel of molecular probes for highly polymorphic loci distributed throughout the genome that are heterozygous in the genome of the individual from whom the cultured cells had been derived. Loss of heterozygosity and the appearance of new alleles will signal the generation of DNA-base-sequence alterations. The observations will be complementary to those being made by others identifying the molecular changes present in human cancers, the hypothesis being that some type(s) of mutations occurring in diploid, non-neoplastic BS cells and in normal cells at a much lower frequency are the changes that constitute the first step in neoplastic transformation.